Another Blues Jr. Crackling Problem

[Enzo]

A signal tracer is just a way to listen to what is inside the amp. It is nothing more than an amplifier with a probe. YOu make the probe and plug it into some other amp you have. Now you can probe some point in your circuit and hear what is there on this other amp.

[Perkinsman]

An arc either goes someplace it shouldn't, or it completes a circuit where it should. If pin 6 arcs to the trace from pin 4, that is the first type. If we solder a pin and it arcs by itself, that would likely be the latter. If we solder a pin to a pad, say, and the pad has cracked free from the trace that trails off, then we can arc between the pad and the trace end. Or maybe the blob of solder on the pin didn't actually wet to the pad, so that arcs.

Paint hides problems, it doesn't fix them. Pin 7 is your plate, and if we are in that second scenario, your paint MIGHT have isolated the plate, leaving only one side of the push pull. I said MIGHT.

In any case, just one working power tube would still pass signal. SO you have no audio. Isolate the problem. Turn up the reverb and shake the amp to crash the springs. Have the master volume up. Does that reverb noise come out? Is the amp out of the cab and no reverb pan? Just touch the tip of the return cable to make a signal.

Are ALL heaters in all five tubes glowing? Is there B+ on pins 1 and 6 of each 12AX7? And on pons 7 and 9 of the power tubes? Wiggle the ribbon cables, aany noise? And with signal applied, does signal momentarily appear?

Signal to input, trace it with scope or signal tracer. Got it at each of those pins 1 and 6?

Heres the voltage results; V1 pin1=140, pin6=11, V2 pin1=0, pin6=135, V4 pin7=314, pin9=292, V5 pin7=313,pin9=291

I moved the amp with the reverb on and master up and heard a faint spring crash but nothing like we typically hear.

[Enzo]

SO when measuring V1 and V2, did you notice the missing B+ on two of the four plates?

[Perkinsman]

SO when measuring V1 and V2, did you notice the missing B+ on two of the four plates?

No, this was the first that I measured the plate voltages. I did change the tubes with new ones but just got similar voltages, although V2 pin1 was still 0. What causes such low voltages if the tubes are good? Would it be bad filter caps?

[Enzo]

Tubes don't cause missing voltages, missing voltages do.

I think V2 pin 1 is an unused triode in V2.

Since pin 6 of V1 has no B+, I'd suspect either R3 is open, or more likely the ribbon cable connection is broken.

[Perkinsman]

Tubes don't cause missing voltages, missing voltages do.

I think V2 pin 1 is an unused triode in V2.

Since pin 6 of V1 has no B+, I'd suspect either R3 is open, or more likely the ribbon cable connection is broken.

V2 pin1 is unused. I decided to replace all 3 plate resistors being use and go over a few bad traces on the back of the board and voila, sound again! In addition, the noise that was initially the problem here is gone too. I had tested R3 early on but I didn't check it's connection on the board, which is probably what the problem was all along. Thanks Enzo for the continued assist & I'll call this one good with your kind help.

One last thing, I had blown F1 and its been replaced but I think the "on" LED lamp went with it. What do I replace it with?

[Enzo]

What to replace an LED with? An LED. The LED is part of a circuit. it could be bad, or it could just be unsoldered. Or the solder connections to either could be bad. The copper traces to and from could be cracked. That LED is on insulated wires. One could have broken off the board in all the moving. And the LED might have just fallen out of the panel hole. In anay case, the LED could be bad, but FIND OUT what the problem us before deciding to replace parts.

[Perkinsman]

It turns out that the amp problem is not resolved yet. I warmed it up & played it a little while this morning and it sounded fine but I noticed V4 wasn't in straight so i gently wiggled it into the socket and smoke from someplace. Turns out R56 opened along with the new LED I had just replaced. I'm just learning how to read schematics but looking at R56 on the schematic, it's connected across the AC heaters, pins 4 & 5 of each tube. Would this indicate a possible loose connection on the V4 pins, ie, maybe a bad trace?

[Jon Snell]

R56 is in series with the LED. Shown on the schematic.

[Jon Snell]

6.3volts is not enough to cause R56 to burn up. I would think there is a solder blob issue on the valve bases causing your issue now.

[Perkinsman]

The led and fuse blew because I had wiggled V4 & it shorted. I tightened the socket pins, cleaned up a few traces, replaced R56, the fuse, the led and powered up. It sounded pretty good but when I tapped V4 or V5, it generated a low frequency hornet sounding buzz that would stop when I touched the tube. It would also go into the oscillation by itself. Anybody experience similar problems? Ideas for a fix?

[Jon Snell]

That sounds like new valves are required, to me.

[g1]

The only tube short that I can think of that could cause that LED to fry is plate to heater.
You are still trying to use the same tube?

[Perkinsman]

I forgot to mention that I had to put in another set of power tubes along with the other things I mentioned before I powered up. I too thought it was a tube issue but it's not, it. This noise is intermittent and i started using my chopstick today to see if i could narrow it down. It seems to start independent of anything I do but it seems to stop by touching the power tubes themselves or the ribbon cable or even just the circuit board the tubes are on.

Tommorow I'm reflowing the board with the sockets since it's about the only thing left.

[Perkinsman]

To bring this post to a close, I pulled both boards & reflowed all joints on each socket & trace repair, cleaned & tightened the pins to each tube. I reinstalled the boards & powered up without noise. I played for over an hour & didn’t have any issues. Plate resistors & solder joints are the Achilles heel of the Hot Rod series.